Rubber composition and pneumatic tire

ABSTRACT

The purpose is to provide rubber compositions and pneumatic tires which have reduced changes in tan δ over time. The present invention relates to rubber compositions containing at least one copolymer obtained by copolymerizing an aromatic vinyl compound and a conjugated diene compound, the rubber compositions having a heat aging resistance index defined by equation (1) of 2.0 or less.

TECHNICAL FIELD

The present invention relates to rubber compositions and pneumatictires.

BACKGROUND ART

Exemplary rubber compositions that have been used in automobile tirescontain diene rubbers such as polybutadiene or butadiene-styrenecopolymers and softeners such as oils.

Moreover, Patent Literature 1 proposes techniques of using hydrogenatedstyrene-butadiene rubbers to improve properties such as abrasionresistance.

CITATION LIST Patent Literature

-   Patent Literature 1: JP H10-218920 A

SUMMARY OF INVENTION Technical Problem

As a result of extensive investigation, the present inventors have foundthat the conventional techniques may allow the rubber tan δ to increaseduring the service of the tire, resulting in a decrease in fuel economy.In other words, it has been found that the conventional techniques leaveroom for improvement in reducing the change in tan δ over time.

The present invention aims to solve the problem and provide rubbercompositions and pneumatic tires which have reduced changes in tan δover time.

Solution to Problem

The present invention relates to rubber compositions, containing atleast one copolymer obtained by copolymerizing an aromatic vinylcompound and a conjugated diene compound,

the rubber compositions having a heat aging resistance index defined bythe following equation (1) of 2.0 or less,

Heat aging resistance index=[(tan δ of rubber composition after heattreatment)−(tan δ of rubber composition before heat treatment)]/(tan δof rubber composition before heat treatment)×100  (1)

wherein each tan δ represents the tan δ at 50° C. of the correspondingrubber composition, and the heat treatment involves allowing the rubbercomposition to stand at a temperature of 90° C. and an oxygenconcentration of 20% for 336 hours.

The heat aging resistance index is preferably 1.5 or less, morepreferably 1.0 or less, still more preferably 0.5 or less, particularlypreferably 0 or less.

Preferably, the rubber compositions further contain at least onesoftener,

the copolymer is at least one hydrogenated styrene-butadiene rubberhaving a weight average molecular weight of 200,000 to 2,000,000 and adegree of hydrogenation of 60 mol % or more, and

the rubber compositions have an A value of less than 4.5 as calculatedfrom Hansen solubility parameters (HSPs) of the hydrogenatedstyrene-butadiene rubber and the softener using the following equation(2):

A=√(α²+β²+γ²)  (2)

wherein α=absolute value of difference between δd of hydrogenatedstyrene-butadiene rubber and δd of softener,β=absolute value of difference between δp of hydrogenatedstyrene-butadiene rubber and δp of softener,γ=absolute value of difference between δh of hydrogenatedstyrene-butadiene rubber and δh of softener,wherein δd: energy from dispersion forces between molecules,δp: energy from dipolar intermolecular forces between molecules,δh: energy from hydrogen bonds between molecules.

An amount of at least one styrene-butadiene rubber is preferably 60% bymass or more based on 100% by mass of at least one rubber component inthe rubber compositions.

The rubber compositions preferably contain, per 100 parts by mass of atleast one rubber component therein, 50 parts by mass or more of at leastone silica.

The rubber compositions preferably contain, per 100 parts by mass of atleast one rubber component therein, 70 parts by mass or less of at leastone silica.

The rubber compositions preferably contain, per 100 parts by mass of atleast one rubber component therein, 30 parts by mass or more of at leastone softener.

An amount of at least one carbon black is preferably 3 parts by mass orless per 100 parts by mass of at least one rubber component in therubber compositions.

The rubber compositions are preferably tread rubber compositions.

The present invention also relates to pneumatic tires, including a tirecomponent at least partially including any of the rubber compositions.

The tire component is preferably a tread.

Advantageous Effects of Invention

The rubber compositions of the present invention contain at least onecopolymer obtained by copolymerizing an aromatic vinyl compound and aconjugated diene compound. Further, the rubber compositions have a heataging resistance index defined as above of 2.0 or less. Such rubbercompositions have reduced changes in tan δ over time.

DESCRIPTION OF EMBODIMENTS

The rubber compositions of the present invention are characterized inthat the rubber compositions contain at least one copolymer obtained bycopolymerizing an aromatic vinyl compound and a conjugated dienecompound, and further, the rubber compositions have a heat agingresistance index defined by the equation (1) below of 2.0 or less. Thus,it is possible to reduce the change in tan δ over time, therebymaintaining good tire performance (fuel economy, etc.) for a long time.Herein, the “reduced changes in tan δ over time” means that the increasein tan δ over time is reduced, and does not mean that the decrease intan δ over time is reduced. This is because a decrease in tan δ overtime indicates better tire performance and thus should not be reduced,but rather should be welcome.

Heat aging resistance index=[(tan δ of rubber composition after heattreatment)−(tan δ of rubber composition before heat treatment)]/(tan δof rubber composition before heat treatment)×100  (1)

wherein each tan δ represents the tan δ at 50° C. of the correspondingrubber composition, and the heat treatment involves allowing the rubbercomposition to stand at a temperature of 90° C. and an oxygenconcentration of 20% for 336 hours.

The rubber compositions have the above-mentioned effect. The reason forthis advantageous effect is not exactly clear, but may be explained asfollows.

To determine whether or not at least a predetermined level of gripperformance can be maintained, it has been conventional to subject arubber composition to heat treatment at 80° C. for two weeks and thenanalyze the change in hardness. However, as a result of extensiveinvestigation, the present inventors have found that the test under theabove-mentioned heat treatment conditions fails to simulate tiresactually in service. As a result of further extensive experimentation onthis problem, the inventors have found that heat treatment performedunder the conditions specified herein can suitably simulate the state oftires actually in service, thus permitting highly correlated evaluationof the change in tan δ over time.

As a result of still further extensive experimentation, the inventorshave found that when a rubber composition shows only a small change intan δ before and after the heat treatment under the conditions specifiedherein, the rubber composition has a reduced change in tan δ over time,even after the tire actually runs 50,000 km. More specifically, it hasbeen found that when a rubber composition has a heat aging resistanceindex of 2.0 or less as calculated using equation (1) with the tan δvalues of the rubber composition before and after the heat treatmentunder the conditions specified herein, the rubber composition has areduced change in tan δ over time, even after the tire actually runs50,000 km.

As described, when a rubber composition has a heat aging resistanceindex of 2.0 or less as calculated using equation (1) with the tan δvalues of the rubber composition before and after the heat treatmentunder the conditions specified herein, the rubber composition has areduced change in tan δ over time.

Accordingly, the present invention solves the problem (purpose) ofreducing the change in tan δ over time by formulating a rubbercomposition which satisfies the parameter of equation (1). Specifically,the parameter does not define the problem (purpose), and the problem tobe solved herein is to reduce the change in tan δ over time. In order toprovide a solution to the problem, a rubber composition is formulated tosatisfy the parameter of equation (1). In other words, the essentialfeature is to satisfy the parameter of equation (1).

Herein, the tan δ of the rubber compositions refers to the tan δ of therubber compositions that have been vulcanized.

The heat treatment is described below.

Herein, the heat treatment involves allowing the (vulcanized) rubbercompositions to stand at a temperature of 90° C. and an oxygenconcentration of 20% for 336 hours.

For example, the heat treatment may be performed using a thermostaticbath whose temperature and oxygen concentration are controllable.Specifically, the vulcanized rubber compositions may be allowed to standin a thermostatic bath set at the above-mentioned temperature and oxygenconcentration for the above-mentioned period of time.

The method for measuring the tan δ is described below.

Herein, the tan δ of the (vulcanized) rubber compositions refers to thetan δ (loss tangent) determined by viscoelastic measurements at ameasurement temperature of 50° C., an initial stain of 10%, a dynamicstrain of 2.5%, and a frequency of 10 Hz.

The heat aging resistance index defined by equation (1) is 2.0 or less,preferably 1.8 or less, more preferably 1.7 or less, still morepreferably 1.5 or less, particularly preferably 1.0 or less, mostpreferably 0.6 or less, further preferably 0.5 or less, furtherpreferably 0 or less, further preferably −0.5 or less, furtherpreferably −1.0 or less, further preferably −1.6 or less, furtherpreferably −2.0 or less, further preferably −3.0 or less, furtherpreferably −3.4 or less, further preferably −3.5 or less, furtherpreferably −4.0 or less, further preferably −4.1 or less, furtherpreferably −4.2 or less, further preferably −4.3 or less, furtherpreferably −4.8 or less, further preferably −5.0 or less, furtherpreferably −5.2 or less, further preferably −5.3 or less, furtherpreferably −5.4 or less, further preferably −6.0 or less, furtherpreferably −7.0 or less, further preferably −8.0 or less, furtherpreferably −9.0 or less, further preferably −10.0 or less, furtherpreferably −11.0 or less, further preferably −12.0 or less, furtherpreferably −13.0 or less.

The lower limit of the heat aging resistance index defined by equation(1) is not limited. A lower index is better.

The tan δ of the rubber compositions (vulcanized rubber compositions)before the heat treatment may appropriately vary within a range thatsatisfies equation (1), and is preferably 0.07 or more, more preferably0.08 or more, still more preferably 0.09 or more, but is preferably 0.50or less, more preferably 0.45 or less, still more preferably 0.40 orless, particularly preferably 0.30 or less, most preferably 0.20 orless.

When the tan δ is within the range indicated above, the advantageouseffect tends to be more suitably achieved, and the properties such aswet grip performance and fuel economy required of tire rubbers also tendto be suitably achieved.

The rubber compositions (vulcanized rubber compositions) before the heattreatment preferably have a stress at 300% elongation (M300) of 6 MPa ormore, more preferably 7 MPa or more, still more preferably 8 MPa ormore, but preferably 30 MPa or less, more preferably 25 MPa or less,still more preferably 20 MPa or less, as measured by subjecting No. 3dumbbell specimens to tensile testing at 23° C. in accordance with JISK6251(2010).

When the M300 is within the range indicated above, the advantageouseffect tends to be more suitably achieved, and the properties such asdurability required of tire rubbers also tend to be suitably achieved.

The rubber compositions (vulcanized rubber compositions) before the heattreatment preferably have a tensile strength at break (TB) of 15 MPa ormore, more preferably 18 MPa or more, still more preferably 20 MPa ormore, but preferably 60 MPa or less, more preferably 50 MPa or less,still more preferably 45 MPa or less, particularly preferably 40 MPa orless, as measured by subjecting No. 3 dumbbell specimens to tensiletesting at 23° C. in accordance with JIS K6251(2010).

When the TB is within the range indicated above, the advantageous effecttends to be more suitably achieved, and the properties such asdurability required of tire rubbers also tend to be suitably achieved.

The rubber compositions (vulcanized rubber compositions) before the heattreatment preferably have an elongation at break (EB) of 250% or more,more preferably 280% or more, still more preferably 320% or more, butpreferably 700% or less, more preferably 650% or less, still morepreferably 600% or less, as measured by subjecting No. 3 dumbbellspecimens to tensile testing at 23° C. in accordance with JISK6251(2010).

When the EB is within the range indicated above, the advantageous effecttends to be more suitably achieved, and the properties such asdurability required of tire rubbers also tend to be suitably achieved.

The heat aging resistance index (tan δ change) defined by equation (1)of the rubber compositions can be controlled by the types and amounts ofthe chemicals (in particular, rubber components, fillers, softeners,silane coupling agents) incorporated in the rubber compositions. Forexample, the tan δ change tends to be reduced by using a rubbercomponent having a few unsaturated bonds, or using a softener highlycompatible with rubber components, or using silica as filler, orreducing the amount of softeners, or using a silane coupling agenthighly reactive with diene rubbers.

Conventional diene rubbers, which have a lot of unsaturated bonds, willundergo changes in tan δ (hardening) with time due to re-crosslinking.In contrast, rubber components having a few unsaturated bonds will beless likely to undergo re-crosslinking because of the small number ofunsaturated bonds and, accordingly, their crosslink density is lesslikely to be changed by the heat generated during the service of thetire. Thus, the change in tan δ over time can be reduced.

Softeners highly compatible with rubber components are less likely tobloom. This can prevent hardening of the rubber over time.

Silica causes less heat build-up than carbon black. This can reduce theprogress of re-crosslinking, thereby reducing the change in tan δ overtime.

Moreover, the change in tan δ can also be controlled by varying theamount of sulfur and vulcanization accelerators.

To be more specific, a heat aging resistance index defined by equation(1) of 2.0 or less can be imparted to a vulcanized rubber composition,for example, by selecting appropriate rubber components, softeners,and/or silica as described later, or by appropriately adjusting theamounts thereof. In particular, such properties may be imparted, forexample, by using a copolymer that has been subjected to hydrogenaddition (hereinafter, also referred to as hydrogenated copolymer) as arubber component, or by using a hydrogenated copolymer and a softenerhighly compatible therewith.

Since the hydrogenated copolymer has a few unsaturated bonds, itscrosslink density is less likely to be changed by the heat generatedduring the service of the tire. Thus, the change in tan δ over time canbe reduced.

Herein, “hydrogenation” means the same as “hydrogen addition”.

The method of using a hydrogenated copolymer and a softener highlycompatible therewith is further described below.

For example, when a softener that is compatible with usual SBR isincorporated with a hydrogenated styrene-butadiene rubber (herein, alsoreferred to as hydrogenated SBR), which has a few double bonds, thesoftener may be incompatible with the hydrogenated SBR and thus bloom.This can allow hardening of the rubber over time to occur easily.

In contrast, when a hydrogenated copolymer is used with a softenerhighly compatible with the hydrogenated copolymer, the softener is lesslikely to bloom. This can prevent hardening of the rubber over time.Moreover, the softener highly compatible with the hydrogenated copolymermay be selected on the basis of its Hansen solubility parameters (HSPs).Specifically, it is sufficient to select a softener having HSPs close tothose of the hydrogenated copolymer (e.g., hydrogenated SBR). Theselected softener is more compatible with the hydrogenated copolymer andthus less likely to bloom. This can prevent hardening of the rubber overtime.

More specifically, to allow a vulcanized rubber composition to have aheat aging resistance index defined by equation (1) of 2.0 or less, itis preferred that the rubber composition contain a softener as well as acopolymer obtained by copolymerizing an aromatic vinyl compound and aconjugated diene compound; the copolymer be a hydrogenatedstyrene-butadiene rubber; and the rubber composition have an A value([(J/cm³)^(1/2)]) of less than 4.5 as calculated from the Hansensolubility parameters (HSPs) of the hydrogenated styrene-butadienerubber and the softener using the equation (2) below. The copolymer ispreferably a hydrogenated styrene-butadiene rubber having a weightaverage molecular weight of 200,000 to 2,000,000 and a degree ofhydrogenation of 60 mol % or more.

The A value is more preferably 4.0 or less, still more preferably 3.5 orless, particularly preferably 3.1 or less, most preferably 2.6 or less,still most preferably 2.1 or less, further most preferably 1.9 or less.The lower limit is not limited and is preferably as close to 0 aspossible. It may be 0.

With the A value indicated above, the heat aging resistance indexdefined by equation (1) can be suitably adjusted to 2.0 or less.

When multiple rubber components or softeners are present, the componentwhich is present in the largest amount is used to calculate the A value.

A=√(α²+β²+γ²)  (2)

wherein α=absolute value of difference between δd of hydrogenatedstyrene-butadiene rubber and δd of softener ([(J/cm³)^(1/2)]),β=absolute value of difference between δp of hydrogenatedstyrene-butadiene rubber and δp of softener ([(J/cm³)^(1/2)]),γ=absolute value of difference between δh of hydrogenatedstyrene-butadiene rubber and δh of softener ([(J/cm³)^(1/2)]),wherein δd: energy from dispersion forces between molecules([(J/cm³)^(1/2)]),δp: energy from dipolar intermolecular forces between molecules([(J/cm³)^(1/2)]),δh: energy from hydrogen bonds between molecules ([(J/cm³)^(1/2)]).

Herein, the Hansen solubility parameters δd, δp, and δh refer to thevalues at 298.15 K and 101.3 kPa determined by the Hansen solubilitysphere method.

As described, in the method of using a hydrogenated copolymer and asoftener highly compatible therewith, it is important to use a softenerhighly compatible with the hydrogenated copolymer. Examples of thesoftener include oils, liquid polymers (liquid diene polymers), andresins as described later. The amount of the softener depends on thetype of softener used.

In the case of a softener having unsaturated bonds that arecross-linkable with the rubber components, specifically a liquid dienepolymer, the amount thereof is not limited.

Conversely, in the case of a softener not having unsaturated bonds thatare cross-linkable with the rubber components, specifically a resin, theamount thereof is preferably 20 parts by mass or less per 100 parts bymass of the rubber components.

Other techniques for adjusting the heat aging resistance index definedby equation (1) to 2.0 or less include reducing the amount of softeners,or using a silane coupling agent highly reactive with diene rubbers.

The tan δ, M300, TB, and EB of the rubber compositions can be controlledby the types and amounts of the chemicals (in particular, rubbercomponents, fillers) incorporated in the rubber compositions. Forexample, the M300, TB, and EB tend to be increased by increasing theamount of fillers or reducing the amount of softeners, while the M300,TB, and EB tend to be decreased by reducing the amount of fillers orincreasing the amount of softeners. For example, the tan δ tends to beincreased by increasing the amount of fillers or the amount ofsofteners, while the tan δ tends to be decreased by reducing the amountof fillers or the amount of softeners.

Examples of techniques for imparting the tan δ, M300, TB, and EB valuesindicated above include the techniques for adjusting the heat agingresistance index defined by equation (1) to 2.0 or less, as well asvarying the type and amount of the filler used.

Chemicals that may be used are described below.

The rubber compositions contain at least one rubber component(hereinafter rubber components) including at least one copolymerobtained by copolymerizing an aromatic vinyl compound and a conjugateddiene compound (hereinafter, also referred to as the copolymer of anaromatic vinyl compound and a conjugated diene compound). The at leastone copolymer may be a single copolymer or a combination of two or morecopolymers.

Since, as described earlier, the change in tan δ over time can be easilyreduced by using a hydrogenated copolymer, the following descriptioncenters on embodiments in which the copolymer is a hydrogenatedcopolymer. It is to be noted, however, that the use of unhydrogenatedcopolymers as the copolymer is not excluded.

The term “rubber components” preferably refer to rubbers having a weightaverage molecular weight (Mw) of 150,000 or more, more preferably350,000 or more. The upper limit of the Mw is not limited and ispreferably 4,000,000 or less, more preferably 3,000,000 or less.

The rubber components preferably include at least one hydrogenatedcopolymer obtained by hydrogenating the conjugated diene portion of thecopolymer of an aromatic vinyl compound and a conjugated diene compound.

The hydrogenated copolymer, in which the number of double bonds has beenreduced by hydrogenation, has a small number of reaction points forre-crosslinking. Thus, the change in tan δ over time can be reduced.

Examples of the aromatic vinyl compound include styrene,α-methylstyrene, 1-vinylnaphthalene, 3-vinyltoluene, ethylvinylbenzene,divinylbenzene, 4-cyclohexylstyrene, and 2,4,6-trimethylstyrene. Thesemay be used alone or in combinations of two or more. Among these,styrene is particularly preferred in view of practical aspects such asavailability of monomers and because the advantageous effect can be moresuitably achieved.

Examples of the conjugated diene compound include 1,3-butadiene,isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene, 2-phenyl-1,3-butadiene,and 1,3-hexadiene. These may be used alone or in combinations of two ormore. Among these, 1,3-butadiene or isoprene is preferred, with1,3-butadiene being more preferred, in view of practical aspects such asavailability of monomers and because the advantageous effect can be moresuitably achieved.

The copolymer of an aromatic vinyl compound and a conjugated dienecompound is preferably a copolymer of styrene and 1,3-butadiene(styrene-butadiene copolymer (SBR)). Therefore, the copolymer ispreferably a styrene-butadiene copolymer, more preferably a hydrogenatedstyrene-butadiene copolymer.

The styrene-butadiene copolymer may be produced by copolymerizingstyrene and 1,3-butadiene in any order. The copolymerization may berandom copolymerization or block copolymerization, preferably randomcopolymerization. The same applies to aromatic vinyl compound/conjugateddiene compound copolymers other than the styrene-butadiene copolymer.

The degree of hydrogenation of the copolymer (the degree ofhydrogenation of the conjugated diene portion of the copolymer of anaromatic vinyl compound and a conjugated diene compound) is preferably60 mol % or more, more preferably 75 mol % or more, still morepreferably 80 mol % or more, particularly preferably 90 mol % or more,most preferably 93 mol % or more. The degree of hydrogenation of thecopolymer is also preferably 99 mol % or less, more preferably 98% mol %or less. When the degree of hydrogenation is within the range indicatedabove, the advantageous effect tends to be more suitably achieved.

The degree of hydrogenation may be calculated from the rate of decreasein the unsaturated bond signals in the ¹H-NMR spectrum.

The copolymer preferably has a weight average molecular weight (Mw) of200,000 or more, more preferably 400,000 or more. The Mw of thecopolymer is also preferably 2,000,000 or less, more preferably1,000,000 or less, still more preferably 800,000 or less, particularlypreferably 600,000 or less. When the Mw is within the range indicatedabove, the advantageous effect tends to be more suitably achieved.

Herein, the weight average molecular weight (Mw) and number averagemolecular weight (Mn) may be determined by gel permeation chromatography(GPC) (GPC-8000 series available from Tosoh Corporation, detector:differential refractometer, column: TSKGEL SUPERMULTIPORE HZ-M availablefrom Tosoh Corporation) calibrated with polystyrene standards.

The copolymer preferably has a glass transition temperature (Tg) of −45°C. or higher, more preferably −35° C. or higher, still more preferably−30° C. or higher, further more preferably −25° C. or higher,particularly preferably −24.5° C. or higher, most preferably −24° C. orhigher. The Tg of the copolymer is also preferably lower than −10° C.,more preferably lower than −12.5° C., still more preferably lower than−13° C., further more preferably lower than −15° C., particularlypreferably lower than −17.5° C., most preferably lower than −20° C. Whenthe Tg is within the range indicated above, the advantageous effecttends to be more suitably achieved.

The glass transition temperature (Tg) of the copolymer is measured asdescribed later in EXAMPLES.

In the case where the copolymer is a styrene-butadiene copolymer, thestyrene content of the styrene-butadiene copolymer is preferably 5% bymass or higher, more preferably 10% by mass or higher, still morepreferably 15% by mass or higher, particularly preferably 20% by mass orhigher, most preferably 25% by mass or higher. The styrene content ofthe styrene-butadiene copolymer is also preferably 40% by mass or lower,more preferably 35% by mass or lower. When the styrene content is withinthe range indicated above, the advantageous effect tends to be moresuitably achieved.

The styrene content is determined as described later in EXAMPLES.

The copolymer may be an unmodified copolymer or a modified copolymer.

The modified copolymer may be any copolymer having a functional groupinteractive with a filler such as silica. For example, it may be a chainend-modified copolymer obtained by modifying at least one chain end of acopolymer with a compound (modifier) having the functional group (achain end-modified copolymer terminated with the functional group); abackbone-modified copolymer having the functional group in the backbone;a backbone- and chain end-modified copolymer having the functional groupin both the backbone and chain end (for example, a backbone- and chainend-modified copolymer in which the backbone has the functional groupand at least one chain end is modified with the modifier); or a chainend-modified copolymer that has been modified (coupled) with apolyfunctional compound having two or more epoxy groups in the moleculeso that a hydroxy or epoxy group is introduced.

Examples of the functional group include amino, amide, silyl,alkoxysilyl, isocyanate, imino, imidazole, urea, ether, carbonyl,oxycarbonyl, mercapto, sulfide, disulfide, sulfonyl, sulfinyl,thiocarbonyl, ammonium, imide, hydrazo, azo, diazo, carboxyl, nitrile,pyridyl, alkoxy, hydroxy, oxy, and epoxy groups. These functional groupsmay be substituted. Among these, amino (preferably amino whose hydrogenatom is replaced with a C1-C6 alkyl group), alkoxy (preferably C1-C6alkoxy), alkoxysilyl (preferably C1-C6 alkoxysilyl) groups arepreferred.

The copolymer may be synthesized, for example, by polymerizing anaromatic vinyl compound and a conjugated diene compound, andspecifically as described below.

Moreover, the hydrogenated copolymer may be synthesized, for example, bypolymerizing an aromatic vinyl compound and a conjugated diene compoundto obtain a polymer, and then hydrogenating the polymer, andspecifically as described below.

<Method for Producing Copolymer> (Polymerization Method)

The copolymer of an aromatic vinyl compound and a conjugated dienecompound may be produced by any polymerization method, includingsolution polymerization, vapor phase polymerization, and bulkpolymerization, particularly preferably by solution polymerization.Moreover, the polymerization may be carried out either in a batch modeor in a continuous mode.

For solution polymerization, the monomer concentration (the combinedamount of styrene and 1,3-butadine in the case of a styrene-butadienecopolymer) in the solvent is preferably 5% by mass or more, morepreferably 10% by mass or more. The monomer concentration in the solventis also preferably 50% by mass or less, more preferably 30% by mass orless.

(Polymerization Initiator for Anionic Polymerization)

For anionic polymerization, any polymerization initiator may be used,but organic lithium compounds are preferred. Examples of the organiclithium compounds include those containing C2-C20 alkyl groups, such asethyllithium, n-propyllithium, isopropyllithium, n-butyllithium,sec-butyllithium, tert-butyllithium, tert-octyllithium, n-decyllithium,phenyllithium, 2-naphthyllithium, 2-butylphenyllithium,4-phenylbutyllithium, cyclohexyllithium, cyclopentyllithium, andreaction products of diisopropenylbenzene and butyllithium. From thestandpoints of availability, safety, and other aspects, n-butyllithiumor sec-butyllithium is preferred among these.

Moreover, the polymerization reaction may be carried out in the presenceof a compound (R) prepared by mixing at least one of the organic lithiumcompounds mentioned above with a compound (B1) containing a functionalgroup interactive with silica. By performing the polymerization in thepresence of the compound (R), the functional group interactive withsilica can be introduced to the polymerization initiating terminal ofthe copolymer. The copolymer thus obtained has a modified initiationterminal. The term “interactive” herein means that a covalent bond or anintermolecular force weaker than the covalent bond (e.g., anelectromagnetic force between molecules such as an ion-dipoleinteraction, dipole-dipole interaction, hydrogen bond, or van der Waalsforce) can be formed between the molecules. The term “functional groupinteractive with silica” refers to a group having at least one atominteractive with silica, such as nitrogen, sulfur, phosphorus, or oxygenatom.

In particular, the compound (R) is preferably a reaction product of anorganic lithium compound and a nitrogen-containing compound such as asecondary amine compound. Specific examples of the nitrogen-containingcompound include dimethylamine, diethylamine, dipropylamine,dibutylamine, dodecamethyleneimine,N,N′-dimethyl-N′-trimethylsilyl-1,6-diaminohexane, piperidine,pyrrolidine, hexamethyleneimine, heptamethyleneimine, dicyclohexylamine,N-methylbenzylamine, di-(2-ethylhexyl)amine, diallylamine, morpholine,N-(trimethylsilyl)piperazine, N-(tert-butyldimethylsilyl)piperazine, and1,3-ditrimethylsilyl-1,3,5-triazinane. The polymerization in thepresence of the compound (R) may be carried out by preliminarily mixingan organic lithium compound with a compound (B1) to prepare a compound(R) and adding the compound (R) to the polymerization system, followedby polymerization. Alternatively, it may be carried out by adding anorganic lithium compound and a compound (B1) to the polymerizationsystem and mixing them in the polymerization system to prepare acompound (R), followed by polymerization.

(Method for Anionic Polymerization)

The production of the copolymer through anionic polymerization using thepolymerization initiator may be carried out by any conventional method.

Specifically, monomers such as styrene and 1,3-butadiene may beanionically polymerized in an organic solvent inert to the reaction, forexample, a hydrocarbon solvent such as an aliphatic, alicyclic, oraromatic hydrocarbon compound using a polymerization initiator such asbutyllithium, optionally in the presence of a randomizer to produce atarget copolymer such as a styrene-butadiene copolymer.

(Hydrocarbon Solvent for Anionic Polymerization)

The hydrocarbon solvent is preferably a C3-C8 hydrocarbon solvent, andexamples include propane, n-butane, isobutane, n-pentane, isopentane,n-hexane, cyclohexane, propene, 1-butene, isobutene, trans-2-butene,cis-2-butene, 1-pentene, 2-pentene, 1-hexene, 2-hexene, benzene,toluene, xylene, and ethylbenzene. These may be used alone or inadmixtures of two or more.

(Randomizer for Anionic Polymerization)

The randomizer refers to a compound that has the function of controllingthe microstructure of the conjugated diene portion of a copolymer, forexample, increase of 1,2-butadiene units or 3,4-isoprene units, or thefunction of controlling the compositional distribution of monomer unitsin a copolymer, for example, randomization of styrene and butadieneunits in a styrene-butadiene copolymer. The randomizer is not limited,and any known compound commonly and conventionally used as a randomizermay be used. Examples include ethers and tertiary amines, such asdimethoxybenzene, tetrahydrofuran, dimethoxyethane, diethylene glycoldibutyl ether, diethylene glycol dimethyl ether,bistetrahydrofurylpropane, triethylamine, pyridine, N-methylmorpholine,N,N,N′,N′-tetramethylethylenediamine, and 1,2-dipiperidinoethane. Otherexamples include potassium salts such as potassium-t-amylate andpotassium-t-butoxide, and sodium salts such as sodium-t-amylate. Theserandomizers may be used alone or in combinations of two or more. Theamount of the randomizer used per mol of the organic lithium compound ispreferably 0.01 mole equivalents or more, more preferably 0.05 moleequivalents or more. The amount of the randomizer per mol of the organiclithium compound is also preferably 1000 mole equivalents or less, morepreferably 500 mole equivalents or less.

The Tg of the copolymer may be controlled by varying the type and amountof the randomizer used. For example, the Tg of the copolymer may belowered by reducing the amount of tetrahydrofuran.

(Reaction Temperature)

The anionic polymerization may be carried out at any reactiontemperature at which the reaction suitably proceeds. Usually, thereaction temperature is preferably −10° C. to 100° C., more preferably25° C. to 70° C.

(Termination of Reaction)

The anionic polymerization may be terminated by addition of a reactionterminator usually used in this field. Examples of such reactionterminators include polar solvents containing active protons such asacetic acid and alcohols (e.g., methanol, ethanol, isopropanol), andmixtures of the foregoing. Other examples include mixtures of theforegoing polar solvents and non-polar solvents such as hexane orcyclohexane. It is usually sufficient that the amount of the reactionterminator added be about equal to or twice the molar amount of theinitiator for anionic polymerization.

Moreover, modification may also be performed by known techniques.

<Coupling>

In the method for producing the copolymer, a coupling agent may be addedto the hydrocarbon solution of the copolymer at any time from theinitiation of the polymerization of monomers until the polymer isrecovered as described later. The coupling agent may be a compoundrepresented by the following formula (3-1):

R¹ _(a)ML_(4-a)  (3-1)

wherein R¹ represents an alkyl group, an alkenyl group, a cycloalkenylgroup, or an aryl group; M represents a silicon atom or a tin atom; Lrepresents a halogen atom or a hydrocarbyloxy group; and a represents aninteger of 0 to 2.

Examples of the coupling agent of formula (3-1) include silicontetrachloride, methyltrichlorosilane, dimethyldichlorosilane,trimethylchlorosilane, tin tetrachloride, methyltrichlorotin,dimethyldichlorotin, trimethylchlorotin, tetramethoxysilane,methyltrimethoxysilane, dimethoxydimethylsilane, methyltriethoxysilane,ethyltrimethoxysilane, dimethoxydiethylsilane, diethoxydimethylsilane,tetraethoxysilane, ethyltriethoxysilane, and diethoxydiethylsilane.

To enhance the processability of the polymer, the amount of the couplingagent added is preferably 0.03 mol or more, more preferably 0.05 mol ormore, per mol of the alkali metal derived from the alkali metalcatalyst. Moreover, to enhance fuel economy, the amount is preferably0.4 mol or less, more preferably 0.3 mol or less.

<Hydrogenation Method>

In the production of a hydrogenated copolymer, the above-describedcopolymer may be hydrogenated to obtain a hydrogenated copolymer.

The hydrogenation may be carried out by any method under any reactionconditions, including known methods and conditions. Usually, thehydrogenation is carried out at 20 to 150° C. under 0.1 to 10 MPahydrogen pressure in the presence of a hydrogenation catalyst. Thedegree of hydrogenation may be set as desired by changing, for example,the amount of the hydrogenation catalyst, the hydrogen pressure duringthe hydrogenation reaction, or the duration of the reaction. Thehydrogenation catalyst used may usually be a compound containing any ofthe metals of groups 4 to 11 of the periodic table. For example,compounds containing any of the atoms: Ti, V, Co, Ni, Zr, Ru, Rh, Pd,Hf, Re, and Pt can be used as hydrogenation catalysts. More specificexamples of the hydrogenation catalysts include metallocene compoundscontaining Ti, Zr, Hf, Co, Ni, Pd, Pt, Ru, Rh, Re, or other metals;supported heterogeneous catalysts in which a metal such as Pd, Ni, Pt,Rh, or Ru is supported on a carrier such as carbon, silica, alumina, ordiatomaceous earth; homogeneous Ziegler catalysts in which an organicsalt or acetylacetone salt of a metal element such as Ni or Co iscombined with a reducing agent such as an organoaluminum; organometalliccompounds or complexes containing Ru, Rh, or other metals; andfullerenes and carbon nanotubes in which hydrogen is stored.

Among these, metallocene compounds containing Ti, Zr, Hf, Co, or Ni arepreferred because they allow the hydrogenation reaction to be carriedout in a homogeneous system in an inert organic solvent. More preferredare metallocene compounds containing Ti, Zr, or Hf. In particular,hydrogenation catalysts obtained by reaction of titanocene compounds andalkyllithiums are preferred because such catalysts are inexpensive andindustrially very useful. Specific examples include hydrogenationcatalysts described in, for example, JP H1-275605 A, JP H5-271326 A, JPH5-271325 A, JP H5-222115 A, JP H11-292924 A, JP 2000-37632 A, JPS59-133203 A, JP S63-5401 A, JP S62-218403 A, JP H7-90017 A, JPS43-19960 B, and JP S47-40473 B. These hydrogenation catalysts may beused alone or in combinations of two or more.

The amount of the copolymers (preferably hydrogenated copolymers) basedon 100% by mass of the rubber components is preferably 60% by mass ormore, more preferably 70% by mass or more, still more preferably 80% bymass or more, particularly preferably 90% by mass or more, mostpreferably 100% by mass. When the amount is within the range indicatedabove, the advantageous effect tends to be more suitably achieved.

The amount of the styrene-butadiene rubbers (preferably hydrogenatedstyrene-butadiene rubbers) based on 100% by mass of the rubbercomponents is preferably 60% by mass or more, more preferably 70% bymass or more, still more preferably 80% by mass or more, particularlypreferably 90% by mass or more, most preferably 100% by mass. When theamount is within the range indicated above, the advantageous effecttends to be more suitably achieved.

Preferably, the rubber compositions contain at least onestyrene-butadiene rubber (preferably, hydrogenated styrene-butadienerubber) in an amount of 60% by mass or more based on 100% by mass of therubber components, and also contain at least one silica in an amount of50 parts by mass or more, but preferably not more than 70 parts by mass,per 100 parts by mass of the rubber components. Moreover, the rubbercompositions preferably contain 30 parts by mass of more of at least onesoftener per 100 parts by mass of the rubber components. Moreover, theamount of at least one carbon black is preferably 3 parts by mass orless per 100 parts by mass of the rubber components.

Examples of additional rubber components that may be used in addition tothe above-described copolymer include diene rubbers such aspolybutadiene rubbers (BR), isoprene-based rubbers,acrylonitrile-butadiene rubbers (NBR), chloroprene rubbers (CR), andbutyl rubbers (IIR). These rubber components may be used alone or incombinations of two or more.

Any BR commonly used in the tire industry may be used. These may be usedalone or in combinations of two or more.

The BR may be commercially available from Ube Industries, Ltd., JSRCorporation, Asahi Kasei Corporation, Zeon Corporation, etc.

Examples of the isoprene-based rubbers include natural rubbers (NR),polyisoprene rubbers (IR), refined NR, modified NR, and modified IR. TheNR may be one commonly used in the tire industry such as SIR20, RSS #3,or TSR20. Non-limiting examples of the IR include those commonly used inthe tire industry such as IR2200. Examples of the refined NR includedeproteinized natural rubbers (DPNR) and highly purified natural rubbers(UPNR). Examples of the modified NR include epoxidized natural rubbers(ENR), hydrogenated natural rubbers (HNR), and grafted natural rubbers.Examples of the modified IR include epoxidized polyisoprene rubbers,hydrogenated polyisoprene rubbers, and grafted polyisoprene rubbers.These may be used alone or in combinations of two or more.

The rubber compositions may contain at least one silica.

Examples of the silica include dry silica (silicic anhydride) and wetsilica (hydrous silicic acid). Wet silica is preferred because itcontains a large number of silanol groups. These may be used alone or incombinations of two or more.

The nitrogen adsorption specific surface area (N₂SA) of the silica is 40m²/g or more, preferably 60 m²/g or more, more preferably 80 m²/g ormore, still more preferably 160 m²/g or more. The N₂SA is alsopreferably 600 m²/g or less, more preferably 300 m²/g or less, stillmore preferably 250 m²/g or less, particularly preferably 200 m²/g orless. When the N₂SA is within the range indicated above, theadvantageous effect tends to be more suitably achieved.

The N₂SA of the silica is determined by the BET method in accordancewith ASTM D3037-81.

The silica may be commercially available from Degussa, Rhodia, TosohSilica Corporation, Solvay Japan, Tokuyama Corporation, etc.

The amount of the silica per 100 parts by mass of the rubber componentsis preferably 30 parts by mass or more, more preferably 40 parts by massor more, still more preferably 50 parts by mass or more, particularlypreferably 55 parts by mass or more, but is preferably 150 parts by massor less, more preferably 100 parts by mass or less, still morepreferably 80 parts by mass or less, particularly preferably 70 parts bymass or less. When the amount is within the range indicated above, theadvantageous effect tends to be better achieved.

The amount of the silica based on 100% by mass of the fillers(reinforcing fillers) in the rubber compositions is preferably 60% bymass or more, more preferably 70% by mass or more, still more preferably80% by mass or more, particularly preferably 90% by mass or more, mostpreferably 100% by mass. When the amount is within the range indicatedabove, the advantageous effect tends to be more suitably achieved.

The rubber compositions containing the silica preferably further containat least one silane coupling agent.

Any silane coupling agent may be used, and examples include sulfidesilane coupling agents such as bis(3-triethoxysilylpropyl)tetrasulfide,bis(2-triethoxysilylethyl)tetrasulfide,bis(4-triethoxysilylbutyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(2-trimethoxysilylethyl)tetrasulfide,bis(2-triethoxysilylethyl)trisulfide,bis(4-trimethoxysilylbutyl)trisulfide,bis(3-triethoxysilylpropyl)disulfide,bis(2-triethoxysilylethyl)disulfide,bis(4-triethoxysilylbutyl)disulfide,bis(3-trimethoxysilylpropyl)disulfide,bis(2-trimethoxysilylethyl)disulfide,bis(4-trimethoxysilylbutyl)disulfide,3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, and3-triethoxysilylpropyl methacrylate monosulfide; mercapto silanecoupling agents such as 3-mercaptopropyltrimethoxysilane and2-mercaptoethyltriethoxysilane; vinyl silane coupling agents such asvinyltriethoxysilane and vinyltrimethoxysilane; amino silane couplingagents such as 3-aminopropyltriethoxysilane and3-aminopropyltrimethoxysilane; glycidoxy silane coupling agents such asγ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane;nitro silane coupling agents such as 3-nitropropyltrimethoxysilane and3-nitropropyltriethoxysilane; and chloro silane coupling agents such as3-chloropropyltrimethoxysilane and 3-chloropropyltriethoxysilane.Commercial products available from Degussa, Momentive, Shin-EtsuSilicone, Tokyo Chemical Industry Co., Ltd., AZmax. Co., Dow CorningToray Co., Ltd., etc. may be used. These may be used alone or incombinations of two or more. Among these, sulfide and mercapto silanecoupling agents are preferred because the advantageous effect tends tobe better achieved. More preferred are disulfide silane coupling agentshaving disulfide bonds such as bis(3-triethoxysilylpropyl)disulfide.

The amount of the silane coupling agents per 100 parts by mass of thesilica is preferably 3 parts by mass or more, more preferably 5 parts bymass or more, but is preferably 20 parts by mass or less, morepreferably 15 parts by mass or less. When the amount is within the rangeindicated above, the advantageous effect tends to be better achieved.

The rubber compositions may contain at least one carbon black.

Non-limiting examples of the carbon black include N134, N110, N220,N234, N219, N339, N330, N326, N351, N550, and N762. These may be usedalone or in combinations of two or more.

The nitrogen adsorption specific surface area (N₂SA) of the carbon blackis preferably 80 m²/g or more, more preferably 100 m²/g or more, but ispreferably 150 m²/g or less, more preferably 130 m²/g or less. When theN₂SA is within the range indicated above, the advantageous effect tendsto be better achieved.

Herein, the N₂SA of the carbon black is measured in accordance with JISK6217-2:2001.

The carbon black may be commercially available from Asahi Carbon Co.,Ltd., Cabot Japan K.K., Tokai Carbon Co., Ltd., Mitsubishi ChemicalCorporation, Lion Corporation, NSCC Carbon Co., Ltd., Columbia Carbon,etc.

The incorporation of carbon black may increase heat build-up, possiblyaccelerating the progress of re-crosslinking. Thus, the amount of thecarbon black per 100 parts by mass of the rubber components ispreferably 10 parts by mass or less, more preferably 3 parts by mass orless, still more preferably 1 part by mass or less, particularlypreferably 0 parts by mass. When the amount is within the rangeindicated above, the advantageous effect tends to be more suitablyachieved.

The rubber compositions preferably contain at least one softener. Anysoftener may be used. Examples include oils, liquid polymers (liquiddiene polymers), and resins. These softeners may be used alone or incombinations of two or more.

Preferred among the softeners is at least one selected from the groupconsisting of oils, liquid polymers, and resins. More preferred areliquid polymers and/or resins, with combinations of liquid polymers andresins being still more preferred.

Any oil may be used. Examples include conventional oils, includingprocess oils such as paraffinic process oils, aromatic process oils, andnaphthenic process oils, low PCA (polycyclic aromatic) process oils suchas TDAE and MES, vegetable oils, and mixtures thereof. These may be usedalone or in combinations of two or more. Aromatic process oils arepreferred among these. Specific examples of the aromatic process oilsinclude Diana process oil AH series available from Idemitsu Kosan Co.,Ltd.

The oils may be commercially available from Idemitsu Kosan Co., Ltd.,Sankyo Yuka Kogyo K.K., Japan Energy Corporation, Olisoy, H&R, HokokuCorporation, Showa Shell Sekiyu K.K., Fuji Kosan Co., Ltd., etc.

The liquid polymers (liquid diene polymers) refer to diene polymers thatare liquid at room temperature (25° C.)

The liquid diene polymers preferably have a polystyrene equivalentweight average molecular weight (Mw) of 1.0×10³ or more, more preferably3.0×10³ or more, still more preferably 5.0×10³ or more, particularlypreferably 1.0×10⁴ or more, most preferably 2.0×10⁴ or more, butpreferably 2.0×10⁵ or less, more preferably 1.0×10⁵ or less, still morepreferably 5.0×10⁴ or less, particularly preferably 3.5×10⁴ or less, asmeasured by gel permeation chromatography (GPC). When the Mw is withinthe range indicated above, the advantageous effect tends to be moresuitably achieved.

Examples of the liquid diene polymers include (co)polymers of at leastone selected from the group consisting of butadiene, isoprene, styrene,farnesene, and derivatives thereof. These (co)polymers may be used aloneor in combinations of two or more. Exemplary such liquid diene polymersinclude liquid styrene-butadiene copolymers (liquid SBR), liquidpolybutadiene polymers (liquid BR), liquid polyisoprene polymers (liquidIR), and liquid styrene-isoprene copolymers (liquid SIR). Among these,liquid SBR and liquid IR are preferred, with liquid IR being morepreferred.

Moreover, the liquid diene polymers may be hydrogenated.

The liquid polymers may be commercially available from CRAY VALLEY,Kuraray Co., Ltd., etc.

Any resin may be used. Examples include coumarone resins, styreneresins, terpene resins, dicyclopentadiene resins (DCPD resins), C5petroleum resins, C9 petroleum resins, C5C9 petroleum resins,p-t-butylphenol acetylene resins, and acrylic resins. These may be usedalone or in combinations of two or more.

The resins may be hydrogenated.

In particular, preferred are resins which are highly compatible with thehydrogenated copolymer used, specifically resins which have HSPs closeto those of the hydrogenated SBR, more specifically resins which have anA value calculated by equation (2) of less than 4.5, i.e., terpeneresins.

The terpene resins may be any resin having units derived from a terpenecompound. Examples include polyterpenes (resins produced bypolymerization of terpene compounds), terpene aromatic resins (resinsproduced by copolymerization of terpene compounds and aromaticcompounds), and aromatic modified terpene resins (resins obtained bymodification of terpene resins with aromatic compounds). These may beused alone or in combinations of two or more. Polyterpenes are preferredamong these.

The terpene compounds refer to hydrocarbons having a compositionrepresented by (C₅H₈)_(n) or oxygen-containing derivatives thereof, eachof which has a terpene backbone and is classified as, for example, amonoterpene (C₁₀H₁₆), sesquiterpene (C₁₅H₂₄), or diterpene (C₂₀H₃₂).Examples of the terpene compounds include α-pinene, β-pinene, dipentene,limonene, myrcene, alloocimene, ocimene, α-phellandrene, α-terpinene,γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, α-terpineol,β-terpineol, and γ-terpineol. Other examples of the terpene compoundsinclude resin acids (rosin acids) such as abietic acid, neoabietic acid,palustric acid, levopimaric acid, pimaric acid, and isopimaric acid. Inother words, the terpene resins also include rosin resins mainlycontaining rosin acids produced by processing pine resin. Examples ofthe rosin resins include natural rosin resins (polymerized rosins) suchas gum rosins, wood rosins, and tall oil rosins; modified rosin resinssuch as maleic acid-modified rosin resins and rosin-modified phenolresins; rosin esters such as rosin glycerol esters; anddisproportionated rosin resins obtained by disproportionation of rosinresins.

The aromatic compounds may be any compound having an aromatic ring.Examples include phenol compounds such as phenol, alkylphenols,alkoxyphenols, and unsaturated hydrocarbon group-containing phenols;naphthol compounds such as naphthol, alkylnaphthols, alkoxynaphthols,and unsaturated hydrocarbon group-containing naphthols; styrene andstyrene derivatives such as alkylstyrenes, alkoxystyrenes, andunsaturated hydrocarbon group-containing styrenes. Styrene is preferredamong these.

The softening point of the resins is preferably −30° C. or higher, morepreferably 30° C. or higher, still more preferably 60° C. or higher,particularly preferably 80° C. or higher, most preferably 100° C. orhigher. The softening point is also preferably 200° C. or lower, morepreferably 160° C. or lower. When the softening point is within therange indicated above, the advantageous effect tends to be more suitablyachieved.

The softening point of the resins is determined in accordance with JIS K6220-1:2001 using a ring and ball softening point measuring apparatusand defined as the temperature at which the ball drops down.

The resins may be commercially available from Maruzen Petrochemical Co.,Ltd., Sumitomo Bakelite Co., Ltd., Yasuhara Chemical Co., Ltd., TosohCorporation, Rutgers Chemicals, BASF, Arizona Chemical, Nitto ChemicalCo., Ltd., Nippon Shokubai Co., Ltd., JXTG Nippon Oil & EnergyCorporation, Arakawa Chemical Industries, Ltd., Taoka Chemical Co.,Ltd., etc.

The amount of the softeners per 100 parts by mass of the rubbercomponents is preferably 5 parts by mass or more, more preferably 10parts by mass or more, still more preferably 20 parts by mass or more,particularly preferably 30 parts by mass or more. The amount is alsopreferably 60 parts by mass or less, more preferably 50 parts by mass orless, still more preferably 45 parts by mass or less. When the amount iswithin the range indicated above, the advantageous effect tends to bemore suitably achieved.

The amount of the softeners includes the amount of the oils contained inthe rubbers (oil extended rubbers), if used.

The amount of the liquid polymers (liquid diene polymers) is notlimited, but is preferably 5 parts by mass or more, more preferably 10parts by mass or more, still more preferably 20 parts by mass or more.The amount is also preferably 60 parts by mass or less, more preferably55 parts by mass or less, still more preferably 50 parts by mass orless, particularly preferably 45 parts by mass or less. When the amountis within the range indicated above, the advantageous effect tends to bemore suitably achieved.

The amount of the resins is preferably 5 parts by mass or more, morepreferably 8 parts by mass or more. The amount is also preferably 20parts by mass or less, more preferably 15 parts by mass or less, stillmore preferably 10 parts by mass or less. When the amount is within therange indicated above, the advantageous effect tends to be more suitablyachieved.

Since the oils are more likely to bloom and can allow hardening of therubber over time to occur easily, the amount of the oils is preferably 5parts by mass or less, more preferably 3 parts by mass or less, stillmore preferably 1 part by mass or less, particularly preferably 0 partsby mass. When the amount is within the range indicated above, theadvantageous effect tends to be more suitably achieved.

The rubber compositions may contain at least one wax.

Non-limiting examples of the waxes include petroleum waxes such asparaffin waxes and microcrystalline waxes; naturally-occurring waxessuch as plant waxes and animal waxes; and synthetic waxes such aspolymers of ethylene, propylene, or other similar monomers. These may beused alone or in combinations of two or more. Among these, petroleumwaxes are preferred, and paraffin waxes are more preferred.

The waxes may be commercially available from Ouchi Shinko ChemicalIndustrial Co., Ltd., Nippon Seiro Co., Ltd., Seiko Chemical Co., Ltd.,etc.

The amount of the waxes per 100 parts by mass of the rubber componentsis preferably 0.3 parts by mass or more, more preferably 0.5 parts bymass or more, but is preferably 20 parts by mass or less, morepreferably 10 parts by mass or less. When the amount is within the rangeindicated above, the advantageous effect tends to be better achieved.

The rubber compositions may contain at least one antioxidant.

Examples of the antioxidants include naphthylamine antioxidants such asphenyl-α-naphthylamine; diphenylamine antioxidants such as octylateddiphenylamine and 4,4′-bis(α,α′-dimethylbenzyl)diphenylamine;p-phenylenediamine antioxidants such asN-isopropyl-N′-phenyl-p-phenylenediamine,N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, andN,N′-di-2-naphthyl-p-phenylenediamine; quinoline antioxidants such as2,2,4-trimethyl-1,2-dihydroquinoline polymer; monophenolic antioxidantssuch as 2,6-di-t-butyl-4-methylphenol and styrenated phenol; and bis-,tris-, or polyphenolic antioxidants such astetrakis[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane.These may be used alone or in combinations of two or more. Among these,p-phenylenediamine and/or quinoline antioxidants are preferred.

The antioxidants may be commercially available from Seiko Chemical Co.,Ltd., Sumitomo Chemical Co., Ltd., Ouchi Shinko Chemical Industrial Co.,Ltd., Flexsys, etc.

The amount of the antioxidants per 100 parts by mass of the rubbercomponents is preferably 0.5 parts by mass or more, more preferably 1part by mass or more, but is preferably 10 parts by mass or less, morepreferably 5 parts by mass or less. When the amount is within the rangeindicated above, the advantageous effect tends to be better achieved.

The rubber compositions may contain at least one stearic acid.

The stearic acid may be a conventional one, e.g., available from NOFCorporation, Kao Corporation, FUJIFILM Wako Pure Chemical Corporation,or Chiba Fatty Acid Co., Ltd.

The amount of the stearic acid per 100 parts by mass of the rubbercomponents is preferably 0.5 parts by mass or more, more preferably 1part by mass or more, but is preferably 10 parts by mass or less, morepreferably 5 parts by mass or less. When the amount is within the rangeindicated above, the advantageous effect tends to be better achieved.

The rubber compositions may contain at least one zinc oxide.

The zinc oxide may be a conventional one, e.g., available from MitsuiMining & Smelting Co., Ltd., Toho Zinc Co., Ltd., HakusuiTech Co., Ltd.,Seido Chemical Industry Co., Ltd., or Sakai Chemical Industry Co., Ltd.

The amount of the zinc oxide per 100 parts by mass of the rubbercomponents is preferably 0.5 parts by mass or more, more preferably 1part by mass or more, but is preferably 10 parts by mass or less, morepreferably 5 parts by mass or less. When the amount is within the rangeindicated above, the advantageous effect tends to be better achieved.

The rubber compositions may contain at least one sulfur.

Examples of the sulfur include those commonly used in the rubberindustry, such as powdered sulfur, precipitated sulfur, colloidalsulfur, insoluble sulfur, highly dispersible sulfur, and soluble sulfur.These may be used alone or in combinations of two or more.

The sulfur may be commercially available from Tsurumi Chemical IndustryCo., Ltd., Karuizawa sulfur Co., Ltd., Shikoku Chemicals Corporation,Flexsys, Nippon Kanryu Industry Co., Ltd., Hosoi Chemical Industry Co.,Ltd., etc.

The amount of the sulfur per 100 parts by mass of the rubber componentsis preferably 0.1 parts by mass or more, more preferably 0.5 parts bymass or more, but is preferably 10 parts by mass or less, morepreferably 5 parts by mass or less, still more preferably 3 parts bymass or less. When the amount is within the range indicated above, theadvantageous effect tends to be better achieved.

The rubber compositions may contain at least one vulcanizationaccelerator.

Any vulcanization accelerator may be used. Examples include guanidinevulcanization accelerators, sulfenamide vulcanization accelerators,thiazole vulcanization accelerators, thiuram vulcanization accelerators,dithiocarbamate vulcanization accelerators, thiourea vulcanizationaccelerators, and xanthate vulcanization accelerators. These may be usedalone or in combinations of two or more. To better achieve theadvantageous effect, guanidine, sulfenamide, thiazole, and thiuramvulcanization accelerators are preferred among these.

Examples of the guanidine vulcanization accelerators include1,3-diphenylguanidine, 1,3-di-o-tolylguanidine, 1-o-tolylbiguanide, adi-o-tolylguanidine salt of dicatechol borate,1,3-di-o-cumenylguanidine, 1,3-di-o-biphenylguanidine, and1,3-di-o-cumenyl-2-propionylguanidine. These may be used alone or incombinations of two or more. Preferred among these is1,3-diphenylguanidine or 1,3-di-o-tolylguanidine.

Examples of the sulfenamide vulcanization accelerators includeN-cyclohexyl-2-benzothiazolylsulfenamide,N,N-dicyclohexyl-2-benzothiazolylsulfenamide,N-tert-butyl-2-benzothiazolylsulfenamide,N-oxydiethylene-2-benzothiazolylsulfenamide, andN-methyl-2-benzothiazolylsulfenamide. These may be used alone or incombinations of two or more. Preferred among these isN-cyclohexyl-2-benzothiazolylsulfenamide.

Examples of the thiazole vulcanization accelerators include2-mercaptobenzothiazole (MBT), dibenzothiazyl disulfide (MBTS),2-(2,4-dinitrophenyl)mercaptobenzothiazole, and2-(2,6-diethyl-4-morpholinothio)benzothiazole. These may be used aloneor in combinations of two or more. Preferred among these is2-mercaptobenzothiazole or dibenzothiazyl disulfide.

Examples of the thiuram vulcanization accelerators includetetrakis(2-ethylhexyl)thiuram disulfide, tetrabenzylthiuram disulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide, anddipentamethylenethiuram tetrasulfide. These may be used alone or incombinations of two or more. Preferred among these istetrakis(2-ethylhexyl)thiuram disulfide or tetrabenzylthiuram disulfide.

The vulcanization accelerators may be commercially available fromKawaguchi Chemical Industry Co., Ltd., Ouchi Shinko Chemical IndustrialCo., Ltd., Sanshin Chemical Industry Co., Ltd., etc.

The amount of the vulcanization accelerators per 100 parts by mass ofthe rubber components is preferably 1 part by mass or more, morepreferably 2 parts by mass or more, but is preferably 10 parts by massor less, more preferably 7 parts by mass or less, still more preferably5 parts by mass or less. When the amount is within the range indicatedabove, the advantageous effect tends to be better achieved.

In addition to the above-mentioned components, the rubber compositionsmay further contain additives commonly used in the tire industry, suchas organic peroxides, and fillers such as calcium carbonate, talc,alumina, clay, aluminum hydroxide, and mica. The amount of each of suchadditives is preferably 0.1 to 200 parts by mass per 100 parts by massof the rubber components.

The rubber compositions may be prepared, for example, by kneading thecomponents using a rubber kneading machine such as an open roll mill ora Banbury mixer, and then vulcanizing the kneaded mixture.

The kneading conditions are as follows: in a base kneading step ofkneading additives other than vulcanizing agents and vulcanizationaccelerators, the kneading temperature is usually 100 to 180° C.,preferably 120 to 170° C., while in a final kneading step of kneadingvulcanizing agents and vulcanization accelerators, the kneadingtemperature is usually 120° C. or lower, preferably 80 to 110° C. Thecomposition after kneading vulcanizing agents and vulcanizationaccelerators is usually vulcanized by press vulcanization, for example.The vulcanization temperature is usually 140 to 190° C., preferably 150to 185° C. The vulcanization time is usually 5 to 15 minutes.

The rubber compositions may be used in tire components (i.e., as tirerubber compositions), including, for example, treads (cap treads),sidewalls, base treads, undertreads, clinches, bead apexes, breakercushion rubbers, rubbers for carcass cord topping, insulations, chafers,and innerliners, and side reinforcement layers of run-flat tires. Therubber compositions are suitable for use in treads, among others.

The pneumatic tires of the present invention can be produced from theabove-described rubber compositions by usual methods. Specifically, anunvulcanized rubber composition containing additives as needed may beextruded into the shape of a tire component (in particular, a tread (captread)) and then formed and assembled with other tire components in atire building machine in a usual manner to build an unvulcanized tire,which may then be heated and pressurized in a vulcanizer to produce atire.

It is sufficient that the tire component (e.g., tread) of the pneumatictires at least partially include any of the rubber compositions. Theentire tire component may include any of the rubber compositions.

The pneumatic tires are suitable for use as tires for passengervehicles, large passenger vehicles, large SUVs, trucks and buses, ortwo-wheeled vehicles, racing tires, studless winter tires, run-flattires, aircraft tires, mining tires, or other tires.

EXAMPLES

The present invention is specifically described with reference to, butnot limited to, examples below.

The chemicals used in the synthesis and polymerization processes arelisted below. The chemicals were purified as needed by conventionaltechniques.

n-Hexane: a product of Kanto Chemical Co., Inc.

Styrene: a product of Kanto Chemical Co., Inc.

Butadiene: 1,3-butadiene available from Tokyo Chemical Industry Co.,Ltd.

TMEDA: N,N,N′,Nf-tetramethylethylenediamine available from KantoChemical Co., Inc.

n-Butyllithium solution: 1.6 M solution of n-butyllithium in hexaneavailable from Kanto Chemical Co., Inc.

Ethanol: a product of Kanto Chemical Co., Inc.

2,6-Di-tert-butyl-p-cresol: Nocrac 200 available from Ouchi ShinkoChemical Industrial Co., Ltd.

The methods for evaluating the prepared copolymers are collectivelydescribed below.

(Measurement of Degree of Hydrogenation of Conjugated Diene Portion ofCopolymer)

A solution having a concentration of 15% by mass was prepared usingcarbon tetrachloride as a solvent and used to measure a H¹-NMR spectrumat 100 MHz. The degree of hydrogenation was calculated from the rate ofdecrease in the unsaturated bond signals in the H¹-NMR spectrum.

(Measurement of Styrene Content)

A ¹H-NMR spectrum was measured using a JEOL JNM-A 400 NMR device at 25°C. The ratio of the phenyl protons of the styrene unit at 6.5 to 7.2 ppmto the vinyl protons of the butadiene unit at 4.9 to 5.4 ppm wascalculated from the spectrum and used to determine the styrene content.

(Measurement of Weight Average Molecular Weight (Mw) and Number AverageMolecular Weight (Mn))

The weight average molecular weight (Mw) and number average molecularweight (Mn) of each copolymer were determined by gel permeationchromatography (GPC) (GPC-8000 series available from Tosoh Corporation,detector: differential refractometer, column: TSKGEL SUPERMULTIPORE HZ-Mavailable from Tosoh Corporation) calibrated with polystyrene standards.

(Measurement of Glass Transition Temperature (Tg))

The glass transition temperature (Tg) was defined as the glasstransition onset temperature measured using a differential scanningcalorimeter (Q200, TA Instruments, Japan) at a temperature increase rateof 10° C./min in accordance with JIS K 7121.

Copolymer Production Examples Synthesis Example 1 (Synthesis of SBR 1,Degree of Hydrogenation (Hereinafter DH): 0 Mol %)

A sufficiently nitrogen-purged heat-resistant reaction vessel wascharged with 2000 mL of n-hexane, 60 g of styrene, 140 g of butadiene,0.93 g of TMEDA, and 0.45 mmol of n-butyllithium, followed by stirringat 50° C. for five hours to perform a polymerization reaction.Thereafter, the reaction was terminated by addition of ethanol, and 1 gof 2,6-di-tert-butyl-p-cresol was added to the reaction solution,followed by purification by reprecipitation to give SBR 1. The SBR 1 hada weight average molecular weight (Mw) of 490,000 and a styrene contentof 30% by mass.

Synthesis Example 2 (Synthesis of Hydrogenated SBR 1, DH: 95 Mol %)

Hydrogenated SBR 1 was prepared in the same manner as described for SBR1, except that the obtained polymer was hydrogenated. Specifically,after the polymerization conversion reaction of SBR 1, thepolymerization reaction was not terminated by addition of ethanol.Instead, the reaction solution was then stirred for 20 minutes whilesupplying hydrogen gas at a pressure of 0.4 MPa gauge to react theunreacted polymer terminal lithium with hydrogen into lithium hydride.Hydrogenation was performed using a titanocene dichloride-based catalystat a hydrogen gas supply pressure of 0.7 MPa gauge and a reactiontemperature of 90° C. Once the cumulative amount of absorbed hydrogenreached the amount corresponding to the target degree of hydrogenation,the reaction temperature was brought to room temperature, and thehydrogen pressure was returned to an ordinary pressure. Then, thereaction solution was drawn from the reaction vessel and introduced intowater with stirring. The solvent was removed by steam stripping to givehydrogenated SBR 1. The hydrogenated SBR 1 had a degree of hydrogenationof 95 mol % and a weight average molecular weight (Mw) of 450,000.

Synthesis Example 3 (Synthesis of Hydrogenated SBR 2, DH: 80 Mol %)

Hydrogenated SBR 2 was prepared in the same manner as described forhydrogenated SBR 1, except that the cumulative amount of absorbedhydrogen was adjusted so as to correspond to the target degree ofhydrogenation. The hydrogenated SBR 2 had a degree of hydrogenation of80 mol % and a weight average molecular weight (Mw) of 480,000.

Synthesis Example 4 (Synthesis of Hydrogenated SBR 3, DH: 60 Mol %)

Hydrogenated SBR 3 was prepared in the same manner as described forhydrogenated SBR 1, except that the cumulative amount of absorbedhydrogen was adjusted so as to correspond to the target degree ofhydrogenation. The hydrogenated SBR 3 had a degree of hydrogenation of60 mol % and a weight average molecular weight (Mw) of 450,000.

TABLE 1 Hydro- Hydro- Hydro- genated genated genated SBR 1 SBR 1 SBR 2SBR 3 Degree of 0 95 80 60 hydrogenation (mol %) Styrene content 30 3030 30 (% by mass) Butadiene 70 70 70 70 content (% by mass) Weightaverage 490,000 450,000 480,000 450,000 molecular weight (Mw) Mw/Mn 1.181.19 1.22 1.18 Glass transition −30 −31 −30 −30 temperature (Tg, ° C.)

The chemicals used in the examples and comparative examples are listedbelow.

SBR 1: Unhydrogenated SBR synthesized as described above

Hydrogenated SBRs 1 to 3: Hydrogenated SBRs synthesized as describedabove

Silica 1: VN3 (N₂SA: 175 m²/g) available from Evonik Degussa

Silica 2: 115GR (N₂SA: 115 m²/g) available from Solvay Japan

Silica 3: 9000GR (N₂SA: 235 m²/g) available from Evonik Degussa

Softener 1: Diana process AH-24 (aromatic oil) available from IdemitsuKosan Co., Ltd.

Softener 2: PS-32 (mineral oil) available from Idemitsu Kosan Co., Ltd.

Softener 3: SYLVARES SA85 (α-methylstyrene resin (copolymer ofα-methylstyrene and styrene), softening point: 85° C.) available fromArizona chemical

Softener 4: NOVARES C100 (coumarone-indene resin, softening point: 95 to105° C.) available from Rutgers Chemicals

Softener 5: Kuraprene LIR30 (liquid IR, weight average molecular weight:29000) available from Kuraray Co., Ltd.

Softener 6: Sylvatraxx 4150 (polyterpene resin, softening point: 150°C.) available from KRATON

Softener 7: RICON 100 (liquid SBR, styrene content: 20% by mass, vinylcontent: 70% by mass, weight average molecular weight: 4500) availablefrom Sartomer

Softener 8: Dercolyte L120 (polylimonene resin, softening point: 120°C.) available from DRT

Silane coupling agent 1: Si266 (bis(3-triethoxysilyl-propyl)disulfide)available from Evonik Degussa

Silane coupling agent 2: Si69 (bis(3-triethoxysilyl-propyl)tetrasulfide)available from Evonik Degussa

Silane coupling agent 3: Si363(3-[ethoxybis(3,6,9,12,15-pentaoxaoctacosan-1-yloxy)silyl]-1-propanethiol,a compound represented by the following formula) available from EvonikDegussa

Antioxidant: Nocrac 6C(N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine) available fromOuchi Shinko Chemical Industrial Co., Ltd.

Stearic acid: stearic acid beads “TSUBAKI” available from NOFCorporation

Zinc oxide: zinc oxide #3 available from HakusuiTech Co., Ltd.

Wax: Ozoace 0355 available from Nippon Seiro Co., Ltd.

Sulfur: powdered sulfur available from Tsurumi Chemical Industry Co.,Ltd.

Vulcanization accelerator 1: vulcanization accelerator: NOCCELER CZ(N-cyclohexyl-2-benzothiazolylsulfenamide) available from Ouchi ShinkoChemical Industrial Co., Ltd.

Vulcanization accelerator 2: SANCELER TBZTD (tetrabenzylthiuramdisulfide) available from Sanshin Chemical Industry Co., Ltd.

Vulcanization accelerator 3: NOCCELER DT (1,3-di-o-tolylguanidine)available from Ouchi Shinko Chemical Industrial Co., Ltd.

Vulcanization accelerator 4: NOCCELER M-P (2-mercaptobenzothiazole)available from Ouchi Shinko Chemical Industrial Co., Ltd.

Vulcanization accelerator 5: NOCCELER TOT-N(tetrakis(2-ethylhexyl)thiuram disulfide) available from Ouchi ShinkoChemical Industrial Co., Ltd.

Vulcanization accelerator 6: NOCCELER D (1,3-diphenylguanidine)available from Ouchi Shinko Chemical Industrial Co., Ltd.

Examples and Comparative Examples

According to the formulation shown in Table 2, the chemicals other thanthe sulfur and vulcanization accelerators were kneaded for five minutesat 150° C. using a 1.7 L Banbury mixer (Kobe Steel, Ltd.) to give akneaded mixture. Next, the sulfur and vulcanization accelerators wereadded to the kneaded mixture, followed by kneading for five minutes at80° C. using an open roll mill to obtain an unvulcanized rubbercomposition.

The unvulcanized rubber composition was press-vulcanized for 10 minutesat 170° C. to obtain a vulcanized rubber composition.

Separately, the unvulcanized rubber composition prepared as above wasformed into the shape of a cap tread and assembled with other tirecomponents to build an unvulcanized tire. The unvulcanized tire waspress-vulcanized at 170° C. for 10 minutes to prepare a test tire (size:195/65R15).

(Heat Treatment)

Moreover, the vulcanized rubber composition was subjected to heattreatment by allowing it to stand in an oven at a temperature of 90° C.and an oxygen concentration of 20% for 336 hours. Thus, a heat-treatedvulcanized rubber composition was prepared.

The vulcanized rubber compositions, heat-treated vulcanized rubbercompositions, and test tires prepared as above were evaluated asdescribed below. Table 2 shows the results.

(Tan δ)

The tan δ of the vulcanized rubber compositions (specimens) andheat-treated vulcanized rubber compositions (specimens) was measuredusing a viscoelastic spectrometer (VES, Iwamoto Seisakusho Co., Ltd.) ata measurement temperature of 50° C., an initial stain of 10%, a dynamicstrain of 2.5%, and a frequency of 10 Hz.

Five test tires of the same formulation were prepared. Four out of thefive test tires were mounted on each wheel of a vehicle (a front-engine,front-wheel-drive vehicle of 2000 cc displacement made in Japan), andthe vehicle was run 50,000 km. Then, specimens were cut out from thetreads of the test tires after 50,000 km running and the unused testtire. The tan δ of the respective specimens was measured as describedabove. Then, a tan δ change before and after running was calculatedaccording to the equation below. A smaller tan δ change before and afterrunning indicates less change in tan δ over time.

tan δ change before and after running=[(tan δ of specimen cut out fromtest tire after 50,000 km running)−(tan δ of specimen cut out fromunused test tire)]/(tan δ of specimen cut out from unused test tire)×100

(Tensile Testing)

In accordance with JIS K6251(2010), No. 3 dumbbell specimens wereprepared from the vulcanized rubber compositions. The specimens weresubjected to tensile testing at 23° C. to determine the stress at 300%elongation (M300), tensile strength at break (TB), and elongation atbreak (EB).

TABLE 2 Comp. Ex. Comp. Ex Comp. Ex. Comp. Ex. Comp. Ex. 1 2 3 4 5 Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Formulation SBR1 100 (parts byHydrogenated SBR 1 100 100 100 100 100 100 100 100 100 100 mass)Hydrogenated SBR 2 Hydrogenated SBR 3 Silica 1 60 60 60 60 60 60 60 6060 60 60 Silica 2 Silica 3 Softener 1 35 35 Softener 2 35 Softener 3 35Softener 4 35 Softener 5 35 15 25 25 25 25 Softener 6 20 10 10 10 10Softener 7 Softener 8 Silane coupling agent 1 5 5 5 5 5 5 5 5 5 Silanecoupling agent 2 5 Silane coupling agent 3 5 Antioxidant 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 1.5 Stearic acid 2 2 2 2 2 2 2 2 2 2 2 Zincoxide 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Wax 1 1 1 1 1 1 1 1 11 1 Sulfur 1.5 1 1 1 1 1.5 1 2 3 0.8 1 Vulcanization accelerator 1 2 2 22 2 2 2 2 2 1.5 2 Vulcanization accelerator 2 1 1 1 1 2 1 1 1 1 0.5Vulcanization accelerator 3 Vulcanization accelerator 4 Vulcanizationaccelerator 5 Vulcanization accelerator 6 1 1 1 1 1 1 1 1 0.5 0.5Evaluation tan δ before heat treatment 0.118 0.135 0.095 0.258 0.1700.100 0.230 0.169 0.149 0.138 0.114 tan δ after heat treatment 0.1210.139 0.097 0.266 0.181 0.087 0.234 0.162 0.141 0.132 0.110 Heat agingresistance index 2.5 3.0 2.1 3.1 6.5 −13.0 1.7 −4.1 −5.4 −4.3 −3.5 tan δbefore tire running 0.110 0.130 0.087 0.253 0.168 0.098 0.225 0.1610.147 0.133 0.112 tan δ after tire running 0.115 0.136 0.091 0.270 0.1850.087 0.229 0.152 0.138 0.127 0.108 tan δ change before and 4.5 4.6 4.66.7 10.1 −11.2 1.8 −5.6 −6.1 −4.5 −3.6 after running α [(J/cm³)^(1/2)] —0.46 0.56 0.35 3.04 0.45 1.00 0.45 0.45 0.45 0.45 β [(J/cm³)^(1/2)] —4.13 4.30 5.96 6.93 1.22 1.58 1.22 1.22 1.22 1.22 Y [(J/cm³)^(1/2)] —1.85 1.85 2.03 2.24 1.61 0.27 1.61 1.61 1.61 1.61 A [(J/cm³)^(1/2)] —4.55 4.71 6.31 7.89 2.07 1.89 2.07 2.07 2.07 2.07 HSP of softenerAromatic Mineral oil SA85 NOVARES Kuraprene Sylvatraxx KurapreneKuraprene Kuraprene Kuraprene oil C100 LIR 4150 LIR LIR LIR LIR δd[(J/cm³)^(1/2)] 18.65 18.75 17.84 21.23 18.64 17.19 18.64 18.64 18.6418.64 δp [(J/cm³)^(1/2)] 4.51 4.68 6.34 7.31 1.60 1.96 1.60 1.60 1.601.60 δh [(J/cm³)^(1/2)] 0.90 0.90 4.78 4.99 1.14 3.02 1.14 1.14 1.141.14 HSP of polymer DH 95% DH 95% DH 95% DH 95% DH 95% DH 95% DH 95% DH95% DH 95% DH 95% δd [(J/cm³)^(1/2)] 18.19 18.19 18.19 18.19 18.19 18.1918.19 18.19 18.19 18.19 δp [(J/cm³)^(1/2)] 0.38 0.38 0.38 0.38 0.38 0.380.38 0.38 0.38 0.38 δh [(J/cm³)^(1/2)] 2.75 2.75 2.75 2.75 2.75 2.752.75 2.75 2.75 2.75 Other properties M300 [MPa] 6.7 10.4 10.1 13.0 12.58.2 17.9 21.3 24.7 20.1 16.6 TB [MPa] 18.5 29.2 26.1 31.6 28.5 17.7 35.137.9 38.7 36.9 37.6 EB [%] 580 559 522 544 565 484 484 424 364 464 506Comp. Ex. Ex. 7 Ex. 8 Ex. 9 Ex 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex.16 6 Formulation SBR1 (parts by Hydrogenated SBR 1 100 100 100 100 100100 mass) Hydrogenated SBR 2 100 100 Hydrogenated SBR 3 100 100 100Silica 1 60 60 60 60 60 60 60 60 60 Silica 2 70 Silica 3 50 Softener 1Softener 2 Softener 3 Softener 4 Softener 5 25 25 25 25 25 25 25 0 0 0 0Softener 6 10 10 10 10 10 10 10 10 10 10 0 Softener 7 25 25 25 Softener8 35 Silane coupling agent 1 5 5 5 5 5 5 5 5 5 5 5 Silane coupling agent2 Silane coupling agent 3 Antioxidant 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.51.5 1.5 1.5 Stearic acid 2 2 2 2 2 2 2 2 2 2 2 Zinc oxide 2.5 2.5 2.52.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Wax 1 1 1 1 1 1 1 1 1 1 1 Sulfur 1 1 1 11 1 1 3 3 3 3 Vulcanization accelerator 1 2 2 2 2 2 2 2 2 2 2 2Vulcanization accelerator 2 1 1 1 1 1 1 1 1 Vulcanization accelerator 31 Vulcanization accelerator 4 1 Vulcanization accelerator 5 1Vulcanization accelerator 6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5Evaluation tan δ before heat treatment 0.187 0.194 0.189 0.203 0.1680.187 0.186 0.165 0.164 0.165 0.327 tan δ after heat treatment 0.1780.184 0.179 0.196 0.161 0.184 0.185 0.168 0.165 0.166 0.373 Heat agingresistance index −4.8 −5.2 −5.3 −3.4 −4.2 −1.6 −0.5 1.8 0.6 0.6 14.1 tanδ before tire running 0.179 0.192 0.184 0.195 0.166 0.179 0.181 0.1570.159 0.157 0.325 tan δ after tire running 0.170 0.183 0.177 0.187 0.1590.176 0.181 0.160 0.161 0.158 0.384 tan δ change before and −5.0 −4.7−3.8 −4.1 −4.2 −1.7 0.0 1.9 1.3 0.6 18.2 after running α [(J/cm³)^(1/2)]0.45 0.45 0.45 0.45 0.45 0.30 0.27 0.904 0.874 0.724 0.947 β[(J/cm³)^(1/2)] 1.22 1.22 1.22 1.22 1.22 1.31 1.95 0.546 2.714 2.6241.566 Y [(J/cm³)^(1/2)] 1.61 1.61 1.61 1.61 1.61 1.61 2.35 0.024 0.7640.764 3.299 A [(J/cm³)^(1/2)] 2.07 2.07 2.07 2.07 2.07 2.10 3.07 1.062.95 2.83 3.77 HSP of softener Kuraprene Kuraprene Kuraprene KurapreneKuraprene Kuraprene Kuraprene RICON RICON RICON L120 LIR LIR LIR LIR LIRLIR LIR 100 100 100 δd [(J/cm³)^(1/2)] 18.64 18.64 18.64 18.64 18.6418.64 18.64 17.466 17.466 17.466 19.317 δp [(J/cm³)^(1/2)] 1.60 1.601.60 1.60 1.60 1.60 1.60 3.00 3.00 3.00 1.98 δh [(J/cm³)^(1/2)] 1.141.14 1.14 1.14 1.14 1.14 1.14 3.514 3.514 3.514 0.191 HSP of polymer DH95% DH 95% DH 95% DH 95% DH 95% DH 80% DH 60% DH 60% DH 80% DH 95% DH60% δd [(J/cm³)^(1/2)] 18.19 18.19 18.19 18.19 18.19 18.34 18.37 18.3718.34 18.19 18.37 δp [(J/cm³)^(1/2)] 0.38 0.38 0.38 0.38 0.38 0.29 3.553.55 0.29 0.38 3.55 δh [(J/cm³)^(1/2)] 2.75 2.75 2.75 2.75 2.75 2.753.49 3.49 2.75 2.75 3.49 Other properties M300 [MPa] 17.6 16.9 19.6 19.116.9 24.1 23.2 22.6 23.7 24.1 21.3 TB [MPa] 35.1 34.3 56.6 34.8 35.733.8 28.4 29.6 34.2 39.2 33.6 EB [%] 468 524 432 465 546 312 287 322 371446 468

Ex.: Example

Comp. Ex.: Comparative ExampleDH: Degree of hydrogenation

Table 2 shows that the change in tan δ over time was reduced in theexamples which contained a copolymer obtained by copolymerizing anaromatic vinyl compound and a conjugated diene compound and which had aheat aging resistance index defined as above of 2.0 or less.

1. A rubber composition, comprising at least one copolymer obtained bycopolymerizing an aromatic vinyl compound and a conjugated dienecompound, the rubber composition having a heat aging resistance indexdefined by the following equation (1) of 2.0 or less,Heat aging resistance index=[(tan δ of rubber composition after heattreatment)−(tan δ of rubber composition before heat treatment)]/(tan δof rubber composition before heat treatment)×100  (1) wherein each tan δrepresents the tan δ at 50° C. of the corresponding rubber composition,and the heat treatment involves allowing the rubber composition to standat a temperature of 90° C. and an oxygen concentration of 20% for 336hours.
 2. The rubber composition according to claim 1, wherein the heataging resistance index is 1.5 or less.
 3. The rubber compositionaccording to claim 1, wherein the heat aging resistance index is 1.0 orless.
 4. The rubber composition according to claim 1, wherein the heataging resistance index is 0.5 or less.
 5. The rubber compositionaccording to claim 1, wherein the heat aging resistance index is 0 orless.
 6. The rubber composition according to claim 1, wherein the rubbercomposition further comprises at least one softener, the copolymer is atleast one hydrogenated styrene-butadiene rubber having a weight averagemolecular weight of 200,000 to 2,000,000 and a degree of hydrogenationof 60 mol % or more, and the rubber composition has an A value of lessthan 4.5 as calculated from Hansen solubility parameters (HSPs) of thehydrogenated styrene-butadiene rubber and the softener using thefollowing equation (2):A=√(α2+β2+γ2)  (2) wherein α=absolute value of difference between δd ofhydrogenated styrene-butadiene rubber and δd of softener, β=absolutevalue of difference between δp of hydrogenated styrene-butadiene rubberand δp of softener, γ=absolute value of difference between δh ofhydrogenated styrene-butadiene rubber and δh of softener, wherein δd:energy from dispersion forces between molecules, δp: energy from dipolarintermolecular forces between molecules, δh: energy from hydrogen bondsbetween molecules.
 7. The rubber composition according to claim 1,wherein an amount of at least one styrene-butadiene rubber is 60% bymass or more based on 100% by mass of at least one rubber component inthe rubber composition.
 8. The rubber composition according to claim 1,wherein the rubber composition comprises, per 100 parts by mass of atleast one rubber component therein, 50 parts by mass or more of at leastone silica.
 9. The rubber composition according to claim 1, wherein therubber composition comprises, per 100 parts by mass of at least onerubber component therein, 70 parts by mass or less of at least onesilica.
 10. The rubber composition according to claim 1, wherein therubber composition comprises, per 100 parts by mass of at least onerubber component therein, 30 parts by mass or more of at least onesoftener.
 11. The rubber composition according to claim 1, wherein anamount of at least one carbon black is 3 parts by mass or less per 100parts by mass of at least one rubber component in the rubbercomposition.
 12. The rubber composition according to claim 1, whereinthe rubber composition is a tread rubber composition.
 13. A pneumatictire, comprising a tire component at least partially comprising therubber composition according to claim
 1. 14. The pneumatic tireaccording to claim 13, wherein the tire component is a tread.